The use of such printed circuit antennas (patch antennas, dipoles, ring slots, etc.) is increasing in the field of telecommunications.
Depending on the use envisioned (fixed site telecommunications, maritime or aeronautical telecommunications, broadcasting, position finding, relaying, etc.) the selection both of the type of radiating element and of the type of transmission line leads to a compromise involving a significant number of parameters:
suitability for use at radio frequency (RF); PA1 degree of resolution of the technology; PA1 type of interfaces required, connections; PA1 power rating; PA1 cost; PA1 dimensions and mass; etc. PA1 very severe increase in losses; PA1 miniaturization of the radiating elements; PA1 difficulties in connection and implementation. PA1 feed from a coaxial line; PA1 feed in the plane of a microstrip transmission line; PA1 feed by electromagnetic coupling from a microstrip or stripline transmission line. PA1 feed-in through a capacitive skirt implemented with the aid of an outer coaxial conductor sheath; PA1 feed-in through a capacitive pellet on or under the patch. PA1 feeding the radiating elements with two orthogonal polarizations; and PA1 integrating the beam forming network (BFN) circuits within the physical mesh size of the array in such a manner as to realize a module which allows the objectives of polarization purity, passband, efficiency, radiation quality, etc. to be met with acceptable cost and technology. PA1 a radiating element that is fed with two orthogonal polarizations; and PA1 a structure in which each polarization is output at a separate level, thus allowing independent control of the BFN circuits and also complete integration of the distribution assembly under the array of radiating elements, without any need for connecting elements other than those existing between each feed device and its corresponding radiating element.
By integrating all these parameters and developing active antennas it is possible to offer printed circuit antennas as highly attractive and economical solutions for the majority of applications envisioned these days.
This is common practice for applications operating in the L band (1.5-1.6 GHz), in the S band (2 GHz), and in the C band (4-6 GHz), and it is tending to become more and more the case for applications in the K band, at present in the KU band (12.4-18 GHz). However the increase in frequency can only be achieved at the cost of great technological effort on account of the resulting difficult problems:
Many applications require only a single polarization per frequency (linear or circular). In this case crossed polarization specifications are not in general very difficult to adhere to. This is the case with L band usage (aeronautical and maritime), S band (relays), L and S band (position finding). For this kind of application, different kinds of feed can be envisioned, depending on the radiating element involved.
The most commonly used excitation mode for printed circuit antennas are:
The first two approaches have been extensively described and studied in so far as they are both easy to implement a priori and they exhibit similarity of propagation behavior with the radiating element itself, which may be approximated by a microstrip transmission line.
Solutions belonging to the third category mark a step forward in feed technology by de-coupling the radiating element from the main transmission line. The increase in the number of parameters thus makes it possible to obtain better control over the passband performance of the assembly.
A printed circuit antenna can thus be fed with the aid of an orthogonal coaxial line. The basic configuration consists in connecting the central conductor of the coaxial cable to a point under the patch where the impedance corresponds to the impedance of the coaxial cable. In fact, this technique is often insufficient in broad-band applications (.gtoreq.1%) because of the probe effect which is due to the non-zero diameter of the conductor. In order to increase the performance of such a transition, devices have recently been developed to compensate he self-inductance of the probe, namely:
These devices are widely known and described, for example in an article "Conformal microstrip antennas", Robert E. Munson (Microwave Journal, March 1988), which describes several types of microstrip antenna, their applications and their performance.
A printed circuit antenna (patch or dipole) can also be fed by means of a microstrip transmission line. Again, these types of feed are widely known. This feed mode is widely used and does not require any special procedures other than those of printing the patch itself. It is thus possible to feed the radiating elements and to implement the distribution elements in the same surface.
Finally, a printed circuit antenna can be fed by an electromagnetic coupling technique. This feed mode allows RF energy to be transferred from a main transmission line without any contact or mechanical connection between the conductors. Moreover by introducing parameters it makes it possible to obtain better control over the matching capacitances of the antenna. It is possible to feed a dipole or a patch type antenna from microstrip transmission lines. It is also possible to feed a radiating element from a stripline transmission line. This can offer features of interest compared with the electrical conditions of a microstrip, which is an open transmission line.
However, all these widely known implementations become difficult to put into effect for applications requiring the use of dual polarization. In fact, for this kind of application the problems become worse. Very often the basic radiating element is not alone but forms a sub-array and the overall problem consists in:
Solutions of the type using two orthogonal coaxial probes lead to complex architectures for feeding the radiating element and for access to each of the BFN circuits. Regardless of the configuration, at least one single stage coaxial/stripline transition is needed as well as a two-stage transition, which involves increased complexity of technology relative to single polarization, associated moreover with poor intrinsic performance. The coupling between two coaxial probes is typically 20 dB for this type of excitation, thus involving re-radiation problems with crossed polarization to be resolved by subterfuges of introducing special sub-arrays (sequential rotations for example).
In any event, development is not easy because of parasitic phenomena. Moreover the solution requires a large amount of engineering and technological effort.
The object of the invention is to deal with the problem thus defined.